A type of close clearance gap that is often applied in turbo-machinery is the labyrinth seal. Labyrinth seals generate large magnitude, destabilizing forces on turbo-machinery rotors especially when operating in high density fluids. Over the years several alternative labyrinth configurations have been developed in an attempt to reduce the destabilizing forcing function and at the same time increase the damping properties. Honeycomb style seals are an example in this context. These have been quite successfully applied in high pressure, centrifugal compressors especially in gas reinjection applications. Honeycomb style labyrinths have however a very high direct stiffness and are sensitive to actual rotor position and running gap geometry. This has led to several incidents in the industry where corrective actions were required to bring units within specifications.
U.S. Pat. No. 4,350,345 A, U.S. Pat. No. 3,614,112 A, DE 211 478 C and CH 407 168 A respectively disclose labyrinth seals according to the preamble of the independent claim. Any improvement of stability is not described. Document DE 21 43 736 describes a stuffing-box seal having axial interconnections between chambers connected by openings with a space between two seal elements.
Rotation of the high density gas inside the labyrinth is a root cause for the large magnitude, destabilizing forces inside labyrinth seals. These forces have a pronounced effect on the design of high density turbo-machinery: they limit the maximum possible running speed, the maximum number of impellers at the rotor, the maximum process pressure, the minimum diameter of the rotor shaft, and the efficiency or a combination of these.
Besides labyrinth seals the destabilizing forces can occur in any close clearance gap where a fluid fills the gap between stator part and rotor part.